On 06/26/2012 11:29 AM, Thomas Renninger wrote: > On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote: >> On 06/25/2012 07:23 PM, Thomas Renninger wrote: >> >>> On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote: >>>> >>>> Daniel Lezcano noticed that after booting with maxcpus=X, if we online the >>>> remaining cpus by writing: echo 1 > /sys/devices/system/cpu/cpuY/online, then >>>> for the newly onlined cpus, the cpuidle directory is not found under >>>> /sys/devices/system/cpu/cpuY. >>>> >>>> Partly, the reason for this is that acpi restricts the initialization to cpus >>>> within the maxcpus limit. (See commit 75cbfb9 "ACPI: Do not try to set up acpi >>>> processor stuff on cores exceeding maxcpus="). The maxcpus= kernel parameter is >>>> used to restrict the number of cpus brought up during boot. That doesn't mean >>>> that we should hard restrict the bring up of the remaining cpus later on. >>> >>> Sorry, but IMO it exaclty does mean that (adding more general lists for >>> further comments). >>> >>> If you can online more cores than maxcpus= via sysfs, this sounds like a bug. >>> Not the other way around. >>> >>> Compare with Documentation/kernel-parameters.txt: >>> maxcpus= [SMP] Maximum number of processors that an SMP kernel >>> should make use of. maxcpus=n : n >= 0 limits the >>> kernel to using 'n' processors. n=0 is a special case, >>> it is equivalent to "nosmp", which also disables >>> the IO APIC. >>> >>> Chances that you run into more problems are high. >> >> >> Right, I agree on that. So, IMHO, maxcpus=X doesn't mean that the kernel must and >> should forbid any new cpus from coming online, but in the interest of avoiding >> problems/complications in some obscure paths, I guess it makes sense to avoid >> onlining new cpus beyond maxcpus. > > Yep, for such reasons: > - That nobody realizes this to be useful and makes use of it in a productive > environment > - If I see maxcpus=X in a bugreport's dmesg command line, > I want to be sure that's true. > - To enforce that things work as documented > > > Wow, after looking a bit into this I found (Documentation/cpu-hotplug.txt): > > maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using > maxcpus=2 will only boot 2. You can choose to bring the > other cpus later online, read FAQ's for more info. > > Looks like someone already documented this (IMO broken) behavior. > I didn't find further info in the FAQs. > >> In any case, I was just trying to see why the simple removal of the setup_max_cpus >> check in acpi_processor_add() wasn't enough to expose the cpuidle directories under >> the new cpus.. and while debugging that, I came up with this patch. I don't mind >> if this doesn't get picked up. > >> Right, the usecase of why somebody would like to online new cpus beyond maxcpus >> doesn't look all that solid anyway. So I am OK with leaving the code as it is now. > > In the end this is a debug option, I expect everybody is aware of that. > Yep, let's just leave it... In this case, let's remove the intel_idle_cpu_init stuff in acpi_cpu_soft_notify, no ? -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog
